This invention relates to an apparatus for examining documents, in particular, documents of value, identification or security documents, having at least two detector units for detecting light emanating from a document to be examined.
To increase forgery-proofness, identification documents, security documents and documents of value, such as bank notes, are provided with security features or printed with suitable security inks.
Security features or security inks can contain luminescent substances that can be excited to glow e.g. by light, electric fields, radiation or sound. To check authenticity, the documents are excited to glow and the luminescence light emitted by the luminescent substances of the document is detected. With reference to the intensity and/or spectral characteristic of the luminescence light it can then be ascertained whether the document is authentic or counterfeit.
Certain security features or security inks are also distinguished by characteristic reflection and/or transmission behavior in certain spectral regions. If a document of value is imitated with the aid of a color copier, for example, usually only the visible color effect of a printed area can be reproduced. Since customary color particles do not have the spectral behavior in certain, in particular invisible, spectral regions that is characteristic of security features or inks, however, counterfeit documents can generally be recognized by corresponding measurement of their reflection and/or transmission behavior in said spectral regions.
The reliability of statements about the authenticity of the checked documents is highly dependent here on the accuracy with which the spectral characteristic, i.e. color, of the light emanating from a document is analyzed. Such analysis can be effected for example by spectrometers, but these require relatively high technical effort and high production costs.
A simpler solution is therefore to use individual detector units, such as photodiodes or photomultipliers, each with different spectral sensitivity. Depending on the the spectral characteristic of the light emanating from the document, the detector units deliver different detector signals which can then be used for spectral analysis of the light. Apparatuses of this type have the disadvantage, however, that the light detected by the various detector units generally does not come from exactly the same partial area of the document due to parallactic errors. This makes it impossible to reliably assess the color properties of the light emanating from a certain partial area of the document. This is of disadvantage in particular when areas with small extensions, such as individual lines of a printed image, are to be examined for their spectral properties, since in this case even small parallactic errors can lead to especially great inaccuracies in the spectral analysis of the light emanating from the document.
It is the problem of the invention to state an apparatus allowing higher reliability when examining the luminescence, reflection and/or transmission properties of documents, in particular documents of value, identification and security documents.
The problem is solved by providing a scattering element on which the light emanating from the document to be examined is scattered, the scattering element and detector units being disposed such that the scattered light can be detected by the detector units.
The invention is based on the idea of scattering the light emanating from different partial areas of the document by means of a scattering element whereby the light components emanating from the individual partial areas are mixed. Individual detector units disposed side by side can thus detect light having components from the different partial areas of the document. The scattering element causes spatial mixture and homogenization of the light emanating from the document.
The invention permits the detector units to detect the light emanating from a common area of the document equally well. Any parallactic errors which would occur with a laterally shifted assembly of detector units are greatly mitigated by the inventively provided scattering element. From the spectral components of the light emanating from the document detected by the individual detector units, statements about the spectral characteristic of the luminescence, reflection and/or transmission behavior of the document can then be derived with high reliability.
In a preferred embodiment of the invention, the scattering element is formed for diffuse transmission and/or diffuse reflection of the light emanating from the document. Diffuse transmission or reflection is intended to refer here to any substantially nondirected transmission or reflection.